In general, the non-reciprocal circuit device such as an isolator or a circulator has a function to pass a signal only in a transmitting direction, while blocking the transmission of a signal in an opposite direction. Such isolator and circulator have a distribution constant type and a lumped constant type. FIGS. 10 and 11 show the structure of a distribution-constant-type, non-reciprocal circuit device. This non-reciprocal circuit device comprises a metal case 9, a strip line member 8 having three input/output electrodes 8a, 8b, 8c radially extending from a circular center portion 80, which may be called central conductor, a pair of disc-shaped microwave ferrite members 7, 7 sandwiching the strip line member 8 coaxially with the circular center portion 80, and a pair of permanent magnets 4, 4 disposed on both sides of the disc-shaped microwave ferrite members 7, 7 for applying a DC magnetic field thereto. Only one permanent magnet 4 may be used. The metal case 9 is provided on the sidewall with connectors 20 (20a, 20b, 20c), a center terminal 50 of each connector 20a, 20b, 20c being connected to each input/output electrode 8a, 8b, 8c of the strip line member 8.
FIG. 12 shows the appearance of a distribution-constant-type, non-reciprocal circuit device disclosed by JP2003-124711A, and FIG. 13 shows its internal structure. This non-reciprocal circuit device 1 comprises an upper iron plate 3, a permanent magnet 4, a lower iron plate 5, an upper ground plate 6a, two ferrite plates (disc-shaped garnet ferrite members) 7, 7, a central conductor 8 having three input/output electrodes 8a, 8b, 8c radially extending at intervals of 120°, which is sandwiched by the two ferrite plates 7, 7, and a lower ground plate 6b, in this order from above between a metal case 9 and an upper lid 2. The central conductor 8 is usually formed by a thin copper plate of 0.1-0.25 mm, and its three input/output electrodes 8a, 8b, 8c respectively project from three slits formed in the sidewall of the metal case 9, with their tip end portions bent and soldered to a circuit board.
The strip line member 8 comprises a resonance portion (substantially triangular center portion) 80 resonating in a TM110 mode, three input/output electrodes 8a, 8b, 8c radially extending from the resonance portion 80, and impedance converters each as long as λ/4 and disposed between the resonance part 80 and each input/output electrode (branched line) 8a, 8b, 8c for impedance matching. When current is supplied to the strip line member 8, a high-frequency magnetic field is generated from the disc-shaped microwave ferrite members 7, 7 such that it surrounds the strip line member 8. Because the permanent magnet 4 generates a rotating magnetic field in the disc-shaped microwave ferrite members 7, 7, the polarization plane of the high-frequency magnetic field rotates when passing through the planar microwave ferrite members 7, 7, giving an output only to a predetermined branched line 8a, 8b, 8c (exhibiting non-reciprocality).
Increasingly higher demand for size and cost reduction is mounting on such distribution-constant-type, non-reciprocal circuit devices. However, because the size of a planar microwave ferrite member is substantially determined by an operating frequency of the non-reciprocal circuit device, two-dimensional size reduction is difficult. Attempts to reduce the thickness of a non-reciprocal circuit device have thus been conducted by enhancing the performance of permanent magnets, making uniform a magnetic flux by combining one permanent magnet with a magnetic yoke or a pole piece, etc. However, thickness reduction is limited, because the non-reciprocal circuit device has a structure in which a planar microwave ferrite member, a strip line member, and a permanent magnet are stacked.